SUMMARY Cell motility and contractility depend on tightly controlled assembly of the actin cytoskeleton. Formins are a major group of actin assembly proteins that nucleate and processively elongate actin filaments. The formin homology domain-containing protein (FHOD) family of formins assembles actin-based structures in several biological contexts, including cell motility and sarcomere organization. Mammals have two FHOD proteins, FHOD1 and FHOD3, with largely distinct expression patterns and functions. FHOD1 assembles stress fibers that contribute to the adhesion, spreading, and motility of a wide variety of cell types. FHOD3 is predominantly expressed in striated muscle (skeletal and cardiac), where it is required for sarcomere organization. Altered expression of both FHOD1 and FHOD3 is associated with some cancers and cardiomyopathies. Despite the essential role of FHOD family members, and their relevance to several diseases, the mechanisms by which they carry out these functions remain poorly understood. Advances in this area have been largely limited by the inability to observe actin assembly activity with mammalian FHOD1 or FHOD3 in vitro. In contrast, we found that purified Drosophila FHOD potently accelerates actin assembly. Our ability to combine biochemical characterization of actin assembly with the genetic tractability of Drosophila makes this an attractive model system to study FHOD proteins. Drosophila has a single FHOD gene that is expressed in several distinct motile and contractile cell types, including macrophages, muscle, tracheal tip cells, and imaginal discs. How can the single FHOD gene in Drosophila be responsible for the diverse range of actin-based structures in these tissues? Drosophila FHOD has several splice variants, including four variations in its C-terminal tail. Because the tail modifies the activities of several other formins, we hypothesize that alternative splicing of the Drosophila FHOD tail tunes its activity, allowing each isoform to assemble specific actin structures. We will test this hypothesis by comparing the biochemical activities, expression patterns, and functional requirement of each isoform. The results of this work will provide insight into the role of the formin tail in regulating actin assembly activity, with implications for diseases such as cancers and cardiomyopathies, in which the expression or activity of FHOD proteins is altered.